Carbonaceous substance-coated graphite particles, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

ABSTRACT

Provided is carbonaceous substance-coated graphite particles that exhibit excellent battery properties when used as a negative electrode material for a lithium ion secondary battery. The carbonaceous substance-coated graphite particles includes: graphite particles; and carbonaceous coatings covering at least part of surfaces of the graphite particles, and the carbonaceous substance-coated graphite particles have a specific surface area S BET  determined by BET method of 4.0 to 15.0 m 2 /g, and a pore volume V s  of pores with a pore size of 7.8 to 36.0 nm is 0.001 to 0.026 cm 3 /g, and in a pore size distribution graph with the pore size being plotted on a horizontal axis and a dV/dP value obtained by differentiating the pore volume with the pore size being plotted on a vertical axis, a pore size P max  with which the dV/dP value is maximized is 2.5 to 5.5 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2022/029171, filedJul. 28, 2022 which claims priority to Japanese Patent Application No.2021-132663, filed Aug. 17, 2021, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to carbonaceous substance-coated graphiteparticles, a negative electrode for a lithium ion secondary battery, anda lithium ion secondary battery.

BACKGROUND OF THE INVENTION

Conventionally, graphite may be used as a negative electrode materialfor a lithium ion secondary battery.

Patent literature 1 discloses “a method for producing a carbon materialfor a negative electrode active material including: a coating step ofmixing graphite powder and a solid novolac resin, thereafter softeningthe novolac resin and applying a shear force to coat the graphite powderwith the novolac resin, thereby forming granulated powder; a heattreatment step of subjecting the granulated powder to heat treatment inan oxygen-containing atmosphere to obtain heat-treated powder; and abaking step of baking the heat-treated powder in an inert gasatmosphere, thereby obtaining a carbon material for a negative electrodeactive material.” (Claim 1)

PATENT LITERATURE

Patent Literature 1: JP 2016-4691 A

SUMMARY OF THE INVENTION

The present inventors used carbonaceous substance-coated graphiteparticles produced by a conventional method as a negative electrodematerial for a lithium ion secondary battery. As a result, it was foundthat the initial charging-discharging efficiency, the outputcharacteristic (25° C. output resistivity), and the cycle characteristic(hereinafter, also collectively called “battery properties”) wereinsufficient in some cases.

Accordingly, aspects of the present invention have an object to providecarbonaceous substance-coated graphite particles that achieve a goodbalance between the initial charging-discharging efficiency, the outputcharacteristic, and the cycle characteristic, particularly excellentoutput and cycle characteristics, when used as a negative electrodematerial for a lithium ion secondary battery.

The present inventors found, through an earnest study, that employingthe configuration described below enables the achievement of theabove-mentioned object, and aspects of the invention have thus beencompleted.

Specifically, aspects of the present invention provide the following [1]to [6].

[1] Carbonaceous substance-coated graphite particles comprising:graphite particles; and carbonaceous coatings covering at least part ofsurfaces of the graphite particles, wherein the carbonaceoussubstance-coated graphite particles have a specific surface area SBETdetermined by BET method of 4.0 to 15.0 m2/g,

-   -   a pore volume Vs of pores with a pore size of 7.8 to 36.0 nm is        0.001 to 0.026 cm3/g, and in a pore size distribution graph with        the pore size being plotted on a horizontal axis and a dV/dP        value obtained by differentiating the pore volume with the pore        size being plotted on a vertical axis, a pore size Pmax with        which the dV/dP value is maximized is 2.5 to 5.5 nm.

[2] The carbonaceous substance-coated graphite particles according to[1], wherein in a particle size distribution of primary particles thatis obtained using X-ray computed tomography, a volume ratio of primaryparticles with an equivalent spherical diameter of not more than 0.8 μmis 3.0 to 53.0%, and in a particle shape distribution of secondaryparticles that is obtained using X-ray computed tomography, a volumeratio of rod-shaped secondary particles is 2.6 to 65.0%.

[3] The carbonaceous substance-coated graphite particles according to[1] or [2], wherein an amount of the carbonaceous coatings is 0.1 to15.0 parts by mass with respect to 100 parts by mass of the graphiteparticles.

[4] The carbonaceous substance-coated graphite particles according toany one of [1] to [3], wherein the carbonaceous substance-coatedgraphite particles are used as a negative electrode material for alithium ion secondary battery.

[5] A negative electrode for a lithium ion secondary battery containingthe carbonaceous substance-coated graphite particles according to anyone of [1] to [3].

[6] A lithium ion secondary battery including the negative electrodeaccording to [5].

According to aspects of the invention, it is possible to providecarbonaceous substance-coated graphite particles that exhibit excellentbattery properties when used as a negative electrode material for alithium ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a three-dimensional image of a spherical particle.

FIG. 1B is a three-dimensional image of the spherical particle observedfrom a different angle from FIG. 1A.

FIG. 1C is a three-dimensional image of the spherical particle observedfrom a different angle from FIGS. 1A and 1B.

FIG. 1D is a three-dimensional image of the spherical particle observedfrom a different angle from FIGS. 1A, 1B, and 1C.

FIG. 2A is a three-dimensional image of a rod-shaped particle.

FIG. 2B is a three-dimensional image of the rod-shaped particle observedfrom a different angle from FIG. 2A.

FIG. 2C is a three-dimensional image of the rod-shaped particle observedfrom a different angle from FIGS. 2A and 2B.

FIG. 2D is a three-dimensional image of the rod-shaped particle observedfrom a different angle from FIGS. 2A, 2B, and 2C.

FIG. 3A is a three-dimensional image of another secondary particle.

FIG. 3B is a three-dimensional image of the other secondary particleobserved from a different angle from FIG. 3A.

FIG. 3C is a three-dimensional image of the other secondary particleobserved from a different angle from FIGS. 3A and 3B.

FIG. 3D is a three-dimensional image of the other secondary particleobserved from a different angle from FIGS. 3A, 3B, and 3C.

FIG. 4 is a cross-sectional view of a battery for evaluation preparedfor battery property evaluation in Examples and Comparative Examples.

FIG. 5 is an example of a pore size distribution graph.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the description, a range expressed using the form of “(numeral) to(another numeral)” should read as a range including both ends defined bythe numerals. For example, a range expressed as “A to B” includes A andB.

Carbonaceous Substance-coated Graphite Particles

Carbonaceous substance-coated graphite particles according to aspects ofthe present invention include graphite particles and carbonaceouscoatings covering at least part of surfaces of the graphite particles,wherein a specific surface area S_(BET) determined by BET method is 4.0to 15.0 m²/g, a pore volume V_(s) of pores with a pore size of 7.8 to36.0 nm is 0.001 to 0.026 cm³/g, and in a pore size distribution graphwith a pore size being plotted on a horizontal axis and a dV/dP valueobtained by differentiating the pore volume by the pore size beingplotted on a vertical axis, a pore size P_(max) with which the dV/dPvalue is maximized is 2.5 to 5.5 nm.

When the carbonaceous substance-coated graphite particles according toaspects of the invention are used as a negative electrode material for alithium ion secondary battery, battery properties are excellent.

Specific Surface Area S_(BET)

The specific surface area S_(BET) of the carbonaceous substance-coatedgraphite particles according to aspects of the invention determined bythe BET method is not less than 4.0 m²/g because a reaction with anelectrolyte is suppressed, leading to excellent battery properties, andis preferably not less than 4.5 m²/g, more preferably not less than 5.0m²/g, further preferably not less than 6.5 m²/g, and particularlypreferably not less than 7.0 m²/g because the foregoing effect is moreexcellent.

On the other hand, because of the similar reason, the specific surfacearea S_(BET) is preferably not more than 15.0 m²/g, more preferably notmore than 14.0 m²/g, further preferably not more than 13.0 m²/g,particularly preferably not more than 12.5 m²/g, and most preferably notmore than 10.0 m²/g.

The specific surface area S_(BET) is determined through nitrogen gasadsorption according to JIS Z 8830:2013 (Determination of the specificsurface area of powders (solids) by gas adsorption-BET method).

Pore Volume V_(s)

As an index correlating with battery properties when the carbonaceoussubstance-coated graphite particles are used, the inventors of thepresent invention focused on the pore volume determined from thenitrogen adsorption isotherm through the density functional theory (DFT)method.

Then, the inventors found out that the pore volume of pores with a poresize of 7.8 to 36.0 nm serves as a good index correlating with batteryproperties.

Specifically, the pore volume Vs of pores with a pore size of 7.8 to36.0 nm of the carbonaceous substance-coated graphite particlesaccording to aspects of the invention is not less than 0.001 cm³/g,preferably not less than 0.009 cm³/g, and more preferably not less than0.010 cm³/g because battery properties are more excellent.

Because of the similar reason, the carbonaceous substance-coatedgraphite particles according to aspects of the invention have the porevolume V_(s) of not more than 0.026 cm³/g, preferably not more than0.025 cm³/g, more preferably not more than 0.024 cm³/g, and furtherpreferably not more than 0.017 cm³/g.

The pore volume is measured through the DFT method according to JIS Z8831-2 (Analysis of mesopores and macropores by gas adsorption) and JISZ 8831-3 (Analysis of micropores by gas adsorption). The pore volumemeasurement is started from a relative pressure of 5×10⁻² Pa.

Pore Size P_(max)

The inventors further found out that pores with a pore size of less than7.8 nm are derived from amorphous carbon, and in the pore sizedistribution graph with a pore size being plotted on a horizontal axisand a dV/dP value obtained by differentiating the pore volume by thepore size being plotted on a vertical axis, the pore size P_(max) withwhich the dV/dP value is maximized serves as a good index correlatingwith battery properties.

Specifically, the pore size P_(max) of the carbonaceous substance-coatedgraphite particles according to aspects of the invention is not morethan 5.5 nm, and preferably not more than 4.5 nm because batteryproperties are more excellent.

Because of the similar reason, the carbonaceous substance-coatedgraphite particles according to aspects of the invention have the poresize P_(max) of not less than 2.5 nm, preferably not less than 3.5 nm,and more preferably not less than 3.8 nm.

The pore size distribution is determined using a fully automatic gasadsorption analyzer As-iQ (manufactured by Quantachrome Instruments).Specifically, the carbonaceous substance-coated graphite particleshaving undergone pretreatment of vacuum-deaeration in a measuring cellat 300° C. for 2 hours are subjected to adsorption isotherm measurementunder the following conditions.

-   -   Adsorption gas: nitrogen gas    -   Measurement temperature: 77.3K    -   Cell size: pellet cell (1.5 cm³)    -   Measurement relative pressure: 1×10⁻² to 1 Pa

Assuming that the pore shape is a slit pore, the obtained adsorptionisotherm is fitted through the non-local density functional theory(NLDFT) method. Accordingly, the pore size P (unit: nm) and the porevolume V (unit: cm³/g) for each pore size P are calculated, and the poresize distribution graph with the pore size being plotted on a horizontalaxis and the value (dV/dP) obtained by differentiating the pore volumewith the pore size being plotted on a vertical axis is obtained. FIG. 5shows an example of the pore size distribution graph.

Volume Ratio of Fine Grains

The carbonaceous substance-coated graphite particles according toaspects of the invention have, in the particle size distribution of theprimary particles obtained using X-ray computed tomography, the volumeratio of the primary particles with the equivalent spherical diameter ofnot more than 0.8 μm (also called “fine grains” for convenience) ofpreferably 3.0 to 53.0%. With this configuration, battery properties aremore excellent.

Because battery properties are further excellent, the volume ratio ofthe fine grains is more preferably not less than 3.3%, furtherpreferably not less than 5.9%, and particularly preferably not less than9.3%.

On the other hand, because of the similar reason, the volume ratio ofthe fine grains is more preferably not more than 52.0%, furtherpreferably not more than 30.0%, and particularly preferably not morethan 17.0%.

Particle Size Distribution of Primary Particles

A method of obtaining a particle size distribution of the primaryparticles constituting the carbonaceous substance-coated graphiteparticles is described.

In order to find sizes of the primary particles, it is necessary tovisualize the carbonaceous substance-coated graphite particles with ahigh resolution in a non-destructive manner. Hence, the carbonaceoussubstance-coated graphite particles are observed with X-ray computedtomography using a radiation source. More specifically, imaging X-raycomputed tomography is performed using SPring-8 beamline (BL24XU) underthe following conditions.

-   -   X-ray energy: 8 keV    -   Image resolution: 1,248 (H)×2,048 (W) pixels    -   Effective pixel size: 68 nm/pixel    -   Exposure time: 0.5 seconds    -   Number of captured projection images: 1,200    -   Defocus: 0.3 mm

The carbonaceous substance-coated graphite particles as a sample arecharged into a quartz glass capillary (inner diameter: about 0.1 mm) andsubjected to X-ray computed tomography.

After projection images of the carbonaceous substance-coated graphiteparticles are captured, a cross-sectional slice image is reconstructed.Subsequently, a watershed analysis function of a commercial imageanalysis software, ExFact VR (available from Nihon Visual Science, Inc.)is used to separate and individually recognize neighboring primaryparticles, and a volume of each primary particle is calculated. Inaddition, an equivalent spherical diameter of each primary particle isdetermined from the obtained volume. Data of each primary particle isplotted in a graph (horizontal axis: equivalent spherical diameter,vertical axis: volume ratio of each primary particle with respect tototal volume), whereby the particle size distribution of the primaryparticles is obtained.

Volume Ratio of Rod-shaped Particles

The carbonaceous substance-coated graphite particles according toaspects of the invention preferably have, in a particle shapedistribution of secondary particles obtained using X-ray computedtomography, the volume ratio of rod-shaped secondary particles (alsocalled “rod-shaped particles” for convenience) of 2.6 to 65.0%. Withthis configuration, battery properties are more excellent.

Because battery properties are further excellent, the volume ratio ofthe rod-shaped particles is more preferably not less than 5.0%, furtherpreferably not less than 10.0%, further more preferably not less than15.0%, particularly preferably not less than 20.0%, and most preferablynot less than 30.0%.

On the other hand, because of the similar reason, the volume ratio ofthe rod-shaped particles is more preferably not more than 59.0%, furtherpreferably not more than 50.0%, particularly preferably not more than40.0%, and most preferably not more than 36.0%.

Particle Shape Distribution of Secondary Particles

A method of obtaining a particle shape distribution of the secondaryparticles constituting the carbonaceous substance-coated graphiteparticles is described.

In order to find shapes of the secondary particles, it is necessary tovisualize the carbonaceous substance-coated graphite particles with ahigh resolution in a non-destructive manner. Hence, the carbonaceoussubstance-coated graphite particles are observed with X-ray computedtomography using a radiation source. More specifically, projection X-raycomputed tomography is performed using SPring-8 beamline (BL24XU) underthe following conditions.

-   -   X-ray energy: 20 keV    -   Image resolution: 2048 (H)×2,048 (W) pixels    -   Effective pixel size: 325 nm/pixel    -   Exposure time: 0.1 seconds    -   Number of captured projection images: 1800    -   Distance between sample and detector: 10 mm

The carbonaceous substance-coated graphite particles as a sample arecharged into a borosilicate glass capillary (inner diameter: about 0.6mm) and subjected to X-ray computed tomography.

After projection images of the carbonaceous substance-coated graphiteparticles are captured, a cross-sectional slice image is reconstructed.Subsequently, a watershed analysis function of a commercial imageanalysis software, ExFact VR (available from Nihon Visual Science, Inc.)is used to separate and individually recognize neighboring secondaryparticles, and a volume of each secondary particle is calculated.

Next, for each secondary particle, three principal axes of inertiamutually perpendicular to one another are defined, and a barycentricmoment of each of the axes is obtained. Among the three barycentricmoments, the largest moment is determined as L, the smallest moment asS, and the intermediate moment as M. According to the followingdefinitions, particle shapes of respective secondary particles areclassified as the spherical shape, rod shape, and another shape.

Spherical shape: S/L≥0.5, and M/L≥0.5

Rod shape: S/L<0.5, and M/L<0.5

With respect to a total volume of secondary particles, a volume ratio ofsecondary particles classified as the spherical shape (sphericalparticles) and a volume ratio of secondary particles classified as therod shape (rod-shaped particles) are determined. Accordingly, the shapedistribution of the secondary particles is obtained.

Examples of three-dimensional images obtained by image analysis of theX-ray computed tomographic data of the secondary particles are shown.

FIGS. 1A to 1D are three-dimensional images of a spherical particle(S/L=0.79, M/L=0.91).

FIGS. 2A to 2D are three-dimensional images of a rod-shaped particle(S/L=0.11, M/L=0.19).

FIGS. 3A to 3D are three-dimensional images of another secondaryparticle (ellipsoidal particle) (S/L=0.22, M/L=0.88).

In FIGS. 1A to 1D, a single secondary particle is observed at differentobservation angles. The same applies to

FIGS. 2A to 2D and FIGS. 3A to 3D.

Amount of Carbonaceous Coatings

An amount of carbonaceous coatings is correlated with the pore volumeand is, for example, not less than 0.1 parts by mass, preferably notless than 0.3 pars by mass, and more preferably not less than 0.5 partsby mass with respect to 100 parts by mass of graphite particles, becausebattery properties are more excellent.

On the other hand, because of the similar reason, an amount of thecarbonaceous coatings is preferably not more than 15.0 parts by mass,more preferably not more than 13.0 parts by mass, further preferably notmore than 11.0 parts by mass, particularly preferably not more than 8.0parts by mass, and most preferably not more than 5.0 parts by mass withrespect to 100 parts by mass of graphite particles.

An amount of the carbonaceous coatings in the carbonaceoussubstance-coated graphite particles is determine as described below.

First, a residual carbon ratio of a precursor of the carbonaceouscoatings (e.g., novolac-type phenolic resin to be described later) isdetermined. The residual carbon ratio is a ratio (unit: mass %) of aresidual amount of the precursor to a charge amount thereof when theprecursor alone is applied with the same heating history as that of thecarbonaceous substance-coated graphite particles to form thecarbonaceous coatings.

An amount of the carbonaceous coatings is determined based on thedetermined residual carbon ratio of the precursor and an amount ofaddition to be described later.

For instance, a case where an amount of the precursor having a residualcarbon ratio of “34 mass %” to be added is “8.0 parts by mass” withrespect to 100 parts by mass of graphite particles is discussed. In thiscase, an amount of the carbonaceous coatings in the carbonaceoussubstance-coated graphite particles to be obtained should be “2.7 partsby mass” (=8.0×0.34) with respect to 100 parts by mass of graphiteparticles.

Method for Producing Carbonaceous Substance-coated Graphite Particles

Next, an exemplary method of producing the foregoing carbonaceoussubstance-coated graphite particles according to aspects of theinvention (hereinafter, also referred to as simply “production methodaccording to aspects of the invention”) is described.

In the production method according to aspects of the invention, first,resin-adhered graphite particles are obtained by causing a novolac-typephenolic resin to adhere to graphite particles. The resin-adheredgraphite particles are then heated in a non-oxidizing atmosphere at 900to 1,500° C. to carbonize the novolac-type phenolic resin. Accordingly,at least part of surfaces of the graphite particles is covered by thecarbonaceous coatings.

Graphite Particles

Graphite particles used in accordance with aspects of the invention arenot particularly limited, and suitable examples thereof include graphiteparticles (spherically shaped graphite) obtained by processing a rawmaterial into a spherical shape.

This raw material is graphite having a different shape from a sphericalshape (including ellipsoidal shape), such as flake graphite. Thegraphite may be either of natural graphite and artificial graphite,while natural graphite is preferred because of high crystallinity orother reasons.

More specific examples of the method of processing the raw material intoa spherical shape include a method in which the raw material is stirredin the presence of a granulation assisting agent such as an adhesive ora resin, a method in which a mechanical external force is applied to rawmaterials without use of a granulation assisting agent, and a method inwhich the both methods are combined.

Among these, the method in which a mechanical external force is appliedto raw materials without use of a granulation assisting agent ispreferred. This method is described below in more detail.

More specifically, a raw material (such as flake graphite) is pulverizedand granulated through application of a mechanical external force usinga pulverizing apparatus. Accordingly, the raw material is processed intoa spherical shape to obtain spherically shaped graphite.

Examples of a pulverizing apparatus include a rotating ball mill,Opposed Jet Mill (manufactured by Hosokawa Micron Corporation), CurrentJet (manufactured by Nisshin Engineering Inc.), Hybridization System(manufactured by Nara Machinery Co., Ltd.), CF mill (manufactured by UBECorporation), MECHANO FUSION System (manufactured by Hosokawa MicronCorporation), and Theta Composer (manufactured by TOKUJU CORPORATION),and among these, Hybridization System (manufactured by Nara MachineryCo., Ltd.) is preferred.

In accordance with aspects of the invention, it is preferable that whilea plurality of pulverizing apparatuses are installed in series, a rawmaterial sequentially passes through the pulverizing apparatuses. Inother words, the pulverizing apparatuses are preferably installed inseries such that immediately after a raw material has passed through oneof the pulverizing apparatuses, the raw material is pulverized andgranulated in the next pulverizing apparatus.

Number of Pulverizing Apparatuses

Here, the number of pulverizing apparatuses is, for example, 2 or more,preferably 3 or more, more preferably 4 or more, further preferably 5 ormore, and particularly preferably 6 or more.

Meanwhile, the number of pulverizing apparatuses is preferably 10 orless, more preferably 8 or less, and further preferably 7 or less.

Pulverizing Time

Time for pulverizing and granulating the raw material (called“pulverizing time” for convenience) in each pulverizing apparatus ispreferably not less than 8 minutes/apparatus, more preferably not lessthan 13 minutes/apparatus, and further preferably not less than 18minutes/apparatus.

Meanwhile, the pulverizing time in each pulverizing apparatus ispreferably not more than 60 minutes/apparatus, more preferably not morethan 50 minutes/apparatus, and further preferably not more than 40minutes/apparatus.

Total Pulverizing Time

A product of the number of pulverizing apparatuses and the pulverizingtime in each pulverizing apparatus (called “total pulverizing time” forconvenience) is preferably not less than 30 minutes, more preferably notless than 50 minutes, and further preferably not less than 90 minutes.

Meanwhile, the total pulverizing time is preferably not more than 180minutes, and more preferably not more than 160 minutes.

Circumferential Speed of Rotor

The pulverizing apparatus normally includes a rotor built therein.

The rotor in each pulverizing apparatus has a circumferential speed ofpreferably not less than 30 m/second, more preferably not less than 40m/second, and further preferably not less than 60 m/second.

Meanwhile, the circumferential speed of the rotor in each pulverizingapparatus is preferably not more than 100 m/second, and more preferablynot more than 80 m/second.

Total Pulverizing Time x Circumferential Speed of Rotor

A product (unit: m) of the total pulverizing time (unit: second) and thecircumferential speed of the rotor (unit: m/second) is preferably notless than 4,000 m, more preferably not less than 10,000 m, furtherpreferably not less than 50,000 m, particularly preferably not less than80,000 m, and most preferably 100,000 m.

Meanwhile, the product is preferably not more than 800,000 m, morepreferably not more than 650,000 m, and further preferably not more than500,000 m.

An amount of the raw material charged into each pulverizing apparatus ispreferably smaller for easier application of a sheer fore and acompression force to the raw material.

Novolac-type Phenolic Resin

The novolac-type phenolic resin used in accordance with aspects of theinvention is preferably expressed by the following Formula (1).

In Formula (1), P represents an arylene group having a hydroxy group,and m represents an integer of 1 or more.

An example of an arylene group having a hydroxy group represented by Pin Formula (1) is a divalent group derived from a phenol (i.e., aresidue from an aromatic ring constituting a phenol, from which twohydrogen atoms are removed).

While a phenol is not particularly limited, specific examples thereofinclude phenol; an alkyl-substituted phenol such as o-cresol, m-cresol,p-cresol, xylenol or p-t-butyl phenol; an aromatic-substituted phenolsuch as p-phenylphenol; a divalent phenol such as catechol orresorcinol; and naphthol such as α-naphthol or β-naphthol. Among these,phenol is preferred.

An integer of 1 or more represented by m in Formula (1) is notparticularly limited and is appropriately selected depending on, forexample, a weight average molecular weight (Mw) of the novolac-typephenolic resin expressed by Formula (1).

Weight Average Molecular Weight

The weight average molecular weight of the novolac-type phenolic resin(in terms of polystyrene) is preferably 500 to 100,000, more preferably600 to 100,000, further preferably 700 to 80,000, and particularlypreferably 1,000 to 50,000 because the carbonaceous substance-coatedgraphite particles according to aspects of the invention can be easilyobtained.

The weight average molecular weight is determined by measurement usinggel permeation chromatography (GPC) under the following conditions.

Measurement Conditions

Apparatus: “HLC-8220” manufactured by TOSOH Corporation

Detector: “UV-8220” manufactured by TOSOH Corporation, set at wavelengthof 280 nm

Analysis column: “TSK-GEL Super HZ2000,” “TSK-GEL Super HZ3000” and“TSK-GEL Super HZM-M” manufactured by TOSOH Corporation are each used

Eluting solvent: tetrahydrofuran

Column temperature: 40° C.

Resin-adhered Graphite Particles

In the production method according to aspects of the invention, first,the novolac-type phenolic resin is caused to adhere to graphiteparticles. In this manner, obtained are resin-adhered graphite particlesin which the novolac-type phenolic resin is adhered to surfaces ofgraphite particles.

Mixing

An example of a method for causing the novolac-type phenolic resin toadhere to graphite particles is a method involving mixing graphiteparticles with the novolac-type phenolic resin.

The mixing method is not particularly limited, and an example thereof isa method in which graphite particles and the novolac-type phenolic resinbeing in a powder form or having been heated and melted into a liquidform are mixed using a kneader or another apparatus. In this step, aliquid dispersant in which graphite particles are dispersed in adispersion medium may be used. As a kneader, a pressure kneader or atwo-roll mill, for example, may be used.

The novolac-type phenolic resin preferably has a powder form for theeasier spread over surfaces of graphite particles. The average particlediameter (D₅₀) of the novolac-type phenolic resin in a powder form isnot particularly limited and, for example, 1 to 50 μm.

Amount of Addition

An amount of the novolac-type phenolic resin to be added may varydepending on the residual carbon ratio of the novolac-type phenolicresin and is suitably exemplified by the following amounts.

In particular, an amount of the novolac-type phenolic resin to be addedis preferably not less than 0.1 parts by mass, more preferably not lessthan 0.2 parts by mass, and further preferably not less than 0.5 partsby mass with respect to 100 parts by mass of graphite particles.

Meanwhile, an amount of the novolac-type phenolic resin to be added ispreferably not more than 35.0 parts by mass, more preferably not morethan 30.0 parts by mass, further preferably not more than 15.0 parts bymass, particularly preferably not more than 10.0 parts by mass, and mostpreferably not more than 5.0 parts by mass with respect to 100 parts bymass of graphite particles.

Heating of Resin-adhered Graphite Particles

Next, the resin-adhered graphite particles are heated in a non-oxidizingatmosphere at 900 to 1,500° C. By this process, the novolac-typephenolic resin is carbonized to turn into a carbonaceous substance(carbonaceous coating). Accordingly, obtained are the carbonaceoussubstance-coated graphite particles in which at least part of surfacesof graphite particles is covered by the carbonaceous coatings.

Because the irreversible capacity of the carbonaceous coatings isprevented from increasing, the heating temperature is not lower than900° C., preferably not lower than 950° C., and more preferably notlower than 1,000° C.

Meanwhile, because the crystallinity of the carbonaceous coatingsimproves, the heating temperature is not higher than 1,500° C.,preferably not higher than 1,300° C., and more preferably not higherthan 1,200° C.

The heating time is preferably not less than one hour and morepreferably not less than two hours. The upper limit thereof is notparticularly limited and is, for example, 30 hours.

The heating atmosphere is a non-oxidizing atmosphere. This is becausethe carbonaceous coatings would be burned and vanished in an oxidizingatmosphere. Examples of a non-oxidizing atmosphere include a nitrogenatmosphere, an argon atmosphere, a helium atmosphere, and a vacuumatmosphere. A substantially non-oxidizing atmosphere may be achieved byplacing, for example, coke breeze which oxidizes by itself and therebydecreases an oxygen concentration in atmosphere.

Hereinbelow, the carbonaceous substance-coated graphite particlesaccording to aspects of the invention may be referred to as “negativeelectrode material according to aspects of the invention.”

Negative Electrode for Lithium Ion Secondary Battery (NegativeElectrode)

A negative electrode for a lithium ion secondary battery according toaspects of the invention contains the negative electrode materialaccording to aspects of the invention. The negative electrode for alithium ion secondary battery is also simply referred to as “negativeelectrode.”

The negative electrode according to aspects of the invention is preparedas with a normal negative electrode.

For preparation of the negative electrode, it is preferable to use anegative electrode mixture preliminarily prepared by adding a binder tothe negative electrode material according to aspects of the invention.The negative electrode mixture may contain an active material or anelectrically conductive material in addition to the negative electrodematerial according to aspects of the invention.

It is preferable that the binder is chemically and electrochemicallystable against an electrolyte, and for the binder, use may be made of,for example, fluororesin such as polytetrafluoroethylene orpolyvinylidene fluoride; resin such as polyethylene, polyvinyl alcohol,or styrene butadiene rubber; and carboxymethyl cellulose, while two ormore of these can be used in combination.

The binder normally accounts for about 1 to 20 mass % of the totalamount of the negative electrode mixture.

More specifically, first, the negative electrode material according toaspects of the invention is optionally adjusted to a desired particlesize through classification or the like. Thereafter, the negativeelectrode material according to aspects of the invention is mixed withthe binder, and the resulting mixture is dispersed in a solvent toprepare the negative electrode mixture in a paste form. Examples of thesolvent include water, isopropyl alcohol, N-methylpyrrolidone, anddimethylformamide. In the mixing and dispersing processes, a knownagitator, mixer, kneader or the like is used.

The prepared paste is applied on one or both of the surfaces of acurrent collector and dried. This process results in a negativeelectrode mixture layer (negative electrode) that is uniformly andfirmly adhered to the current collector. The negative electrode mixturelayer has a thickness of preferably 10 to 200 μm and more preferably 20to 100 μm.

After the negative electrode mixture layer is formed, compressionbonding such as press pressurization is performed, whereby the adhesionstrength of the negative electrode mixture layer (negative electrode) tothe current collector can be further improved.

The shape of the current collector is not particularly limited, andexamples thereof include a foil-like shape, a mesh shape, and a net-likeshape such as an expanded metal shape. The material of the currentcollector is preferably copper, stainless steel, nickel or the like. Thecurrent collector preferably has a thickness of about 5 to 20 μm in acase of a foil-like shape.

Coated Electrode Density

The negative electrode according to aspects of the invention has thecoated electrode density of preferably not less than 1.10 g/cm³ and morepreferably not less than 1.20 g/cm³, but preferably not more than 2.00g/cm³ and more preferably not more than 1.90 g/cm³.

The coated electrode density of the negative electrode is determined asdescribed below.

A negative electrode having been punched out to have a given area issubjected to measurements of mass (with use of an electronic balance)and a thickness (with use of a micrometer). Subsequently, 10 pieces ofcurrent collector having been punched out to have the same area aresubjected to measurement of mass, and their average value is treated asthe mass of the current collector. Moreover, the thickness of thecurrent collector is determined from the density of metal constitutingthe current collector. The coated electrode density of the negativeelectrode is then determined according to the following equation.

Coated electrode density of negative electrode=(mass of negativeelectrode−mass of current collector)/{(thickness of negativeelectrode−thickness of current collector)×(punched out area)}

Lithium Ion Secondary Battery

A lithium ion secondary battery according to aspects of the inventionincludes the negative electrode according to aspects of the invention.

The lithium ion secondary battery according to aspects of the inventionincludes components such as a positive electrode and a non-aqueouselectrolyte in addition to the negative electrode according to aspectsof the invention. The lithium ion secondary battery according to aspectsof the invention is composed of, for example, a negative electrode, anon-aqueous electrolyte and a positive electrode superposed in thisorder and accommodated in an exterior material.

The type of the lithium ion secondary battery according to aspects ofthe invention can be arbitrarily selected from a cylindrical type, asquare type, a coin type, a button type and other types, depending onthe intended use, the device to which the battery is to be mounted, therequired charging-discharging capacity, or the like.

Positive Electrode

For a material of the positive electrode (positive electrode activematerial), a material that can absorb and store as well as releaselithium in a sufficient amount is preferably selected. Examples of apositive electrode active material include, in addition to lithium, alithium-containing compound such as a lithium-containing transitionmetal oxide, a transition metal chalcogenide, a vanadium oxide or alithium compound thereof; a Chevrel phase compound expressed by FormulaM_(x)Mo₆S_(8-y) (where M represents at least one transition metalelement, X is a numerical value in the range of 0≤X≤4, and Y is anumerical value in the range of 0≤Y≤1); activated carbon; and activatedcarbon fiber. The vanadium oxide is expressed by V₂O₅, V₆O₁₃, V₂O₄ orV₃O₈.

The lithium-containing transition metal oxide is a composite oxide oflithium and a transition metal, and lithium and two or more kinds oftransition metals may be mixed to form a solid solution as thelithium-containing transition metal oxide. A single composite oxide maybe used alone, or two or more composite oxides may be used incombination.

The lithium-containing transition metal oxide is specifically expressedby LiM¹ _(1-X)M² _(X)O₂ (where M¹ and M² represent at least onetransition metal element, and X is a numerical value in the range of0≤X≤1) or LiM¹ _(1-Y)M² _(Y)O₄ (where M¹ and M² represent at least onetransition metal element, and Y is a numerical value in the range of0≤X≤1).

The transition metal element represented by M¹ and M² may be Co, Ni, Mn,Cr, Ti, V, Fe, Zn, Al, In or Sn, with Co, Ni, Fe, Mn, Ti, Cr, V and Albeing preferred. Preferred examples include LiCoO₂, LiNiO₂, LiMnO₂,LiNi_(0.9)Co_(0.1)O₂, and LiNi_(0.5)Co_(0.5)O₂.

Using, for example, lithium, an oxide of a transition metal, a hydroxideof a transition metal, and a salt of a transition metal as startingmaterials, the lithium-containing transition metal oxide is obtained bymixing the starting materials according to the composition of thedesired metal oxide and baking the mixture in an oxygen atmosphere attemperature of 600 to 1,000° C.

As the positive electrode active material, any of the foregoingcompounds may be used alone, or two or more thereof may be used incombination. For instance, a carbonate such as lithium carbonate can beadded to the positive electrode. When the positive electrode is formed,various additives including an electrically conductive agent and abinder that are conventionally known can be suitably used.

The positive electrode is prepared by, for example, coating bothsurfaces of a current collector with a positive electrode mixturecomprising a positive electrode active material, a binder, and anelectrically conductive agent for imparting electrical conductivity tothe positive electrode, thereby forming a positive electrode mixturelayer.

As the binder, a binder used in preparation of a negative electrode canbe used.

As the electrically conductive agent, a conventionally knownelectrically conductive agent such as a graphitized substance or carbonblack is used.

The shape of the current collector is not particularly limited, andexamples thereof include a foil-like shape and a net-like shape. Thematerial of the current collector is aluminum, stainless steel, nickelor the like. The current collector preferably has a thickness of 10 to40 μm.

As with the negative electrode, the positive electrode may be preparedby applying the positive electrode mixture in a paste form to thecurrent collector, drying the applied positive electrode mixture, andperforming compression bonding such as press pressurization.

Non-Aqueous Electrolyte

The non-aqueous electrolyte may be a liquid non-aqueous electrolyte(non-aqueous electrolytic solution), or a polyelectrolyte such as asolid electrolyte or a gel electrolyte.

As the non-aqueous electrolyte, use is made of a lithium salt which isan electrolyte salt used for an ordinary non-aqueous electrolyticsolution, such as LiPF₆, LiBF₄, LiAsF₆, LiClO₄, LiB(C₆H₅), LiCl, LiBr,LiCF₃SO₃, LiCH₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiN(CF₃CH₂OSO₂)₂, LiN(CF₃CF₂OSO₂)₂, LiN (HCF₂CF₂CH₂OSO₂)₂, LiN ((CF₃)₂CHOSO₂)₂,LiB[{C₆H₃(CF₃)₂}]₄, LiAlCl₄, or LiSiF₆. From the oxidative stabilitypoint of view, LiPF₆ and LiBF₄ are preferred.

The electrolyte salt concentration in the non-aqueous electrolyticsolution is preferably 0.1 to 5.0 mol/L and more preferably 0.5 to 3.0mol/L.

Examples of a solvent used to prepare the non-aqueous electrolyticsolution include a carbonate such as ethylene carbonate, propylenecarbonate, dimethyl carbonate or diethyl carbonate; an ether such as1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran,2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolan,4-methyl-1,3-dioxolan, anisole, or diethyl ether; a thioether such assulfolane or methyl sulfolane; a nitrile such as acetonitrile,chloronitrile or propionitrile; and an aprotic organic solvent such astrimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide,N-methylpyrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene,benzoyl chloride, benzoyl bromide, tetrahydrothiophene,dimethylsulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, or dimethylsulfite.

When the non-aqueous electrolyte is a polyelectrolyte such as a solidelectrolyte or a gel electrolyte, a polymer gelated with a plasticizer(non-aqueous electrolytic solution) as a matrix is preferably used.

As a polymer constituting the matrix, use is suitably made of anether-based polymer compound such as polyethylene oxide, or acrosslinked compound thereof; a poly(meth)acrylate-based polymercompound; or a fluorine-based polymer compound such as polyvinylidenefluoride, or vinylidene fluoride-hexafluoropropylene copolymer.

The non-aqueous electrolytic solution serving as a plasticizer has anelectrolyte salt concentration of preferably 0.1 to 5.0 mol/L and morepreferably 0.5 to 2.0 mol/L.

The plasticizer content in the polyelectrolyte is preferably 10 to 90mass % and more preferably 30 to 80 mass %.

Separator

A separator can be also used in the lithium ion secondary batteryaccording to aspects of the invention.

The material of the separator is not particularly limited, and use ismade of, for example, woven fabric, non-woven fabric, and a fine porousfilm made of synthetic resin. Among these, a fine porous film made ofsynthetic resin is preferred, and in particular a polyolefin-based fineporous film is more preferred in terms of the thickness, film strength,and film resistance. Suitable examples of the polyolefin-based fineporous film include a polyethylene fine porous film, a polypropylenefine porous film, and a composite fine porous film thereof.

EXAMPLES

Aspects of the present invention are specifically described below withreference to examples. However, the invention is not limited to theexamples described below.

Preparation of Coating Agent

As a coating agent with which graphite particles are coated,novolac-type phenolic resins A to C were prepared as described below.

Novolac-type Phenolic Resin A

A mixture al was obtained by mixing o-cresol and s-trioxane at a molarratio of 1/1 (o-cresol/s-trioxane).

Acetic acid was added to the mixture al, the resulting mixture washeated in nitrogen to 80° C., and a mixture liquid of sulfuric acid andacetic acid was gradually added dropwise to the resulting mixture,whereby a mixture a2 was obtained.

The mixture a2 was stirred at 110° C. for 3 hours and subsequentlycooled to obtain a reaction mixture a3.

The reaction mixture a3 was put into a 5 mass % aqueous sodium hydrogencarbonate solution to cause a resin to be generated and precipitated.The precipitate was filtered out, washed with warm water, and thereafterdried by air. The precipitate was further dried under reduced pressureat 110° C. for 16 hours, and then pulverized by means of an impact mill.

In this manner, obtained was the novolac-type phenolic resin A in apowder form (particle diameter D₅₀: 34 μm, weight average molecularweight: 2,800, residual carbon ratio: 34 mass %).

Novolac-Type Phenolic Resin B

A mixture b1 was obtained by mixing p-cresol and s-trioxane at a molarratio of 1/1 (p-cresol/s-trioxane).

Acetic acid was added to the mixture b1, the resulting mixture washeated in nitrogen to 80° C., and a mixture liquid of sulfuric acid andacetic acid was gradually added dropwise to the resulting mixture,whereby a mixture b2 was obtained.

The mixture b2 was stirred at 110° C. for 3 hours and subsequentlycooled to obtain a reaction mixture b3.

The reaction mixture b3 was put into a 5 mass % aqueous sodium hydrogencarbonate solution to cause a resin to be generated and precipitated.The precipitate was filtered out, washed with warm water, and thereafterdried by air. The precipitate was further dried under reduced pressureat 110° C. for 16 hours, and then pulverized by means of an impact mill.

In this manner, obtained was the novolac-type phenolic resin B in apowder form (particle diameter D₅₀: 30 μm, weight average molecularweight: 1,550, residual carbon ratio: 23 mass %).

Novolac-Type Phenolic Resin C

A mixture c1 was obtained by mixing o-cresol and s-trioxane at a molarratio of 1/1.2 (o-cresol/s-trioxane).

Acetic acid was added to the mixture c1, the resulting mixture washeated in nitrogen to 80° C., and a mixture liquid of sulfuric acid andacetic acid was gradually added dropwise to the resulting mixture,whereby a mixture c2 was obtained.

The mixture c2 was stirred at 110° C. for 3 hours and subsequentlycooled to obtain a reaction mixture c3.

The reaction mixture c3 was put into a 5 mass % aqueous sodium hydrogencarbonate solution to cause a resin to be generated and precipitated.The precipitate was filtered out, washed with warm water, and thereafterdried by air. The precipitate was further dried under reduced pressureat 110° C. for 16 hours, and then pulverized by means of an impact mill.

In this manner, obtained was the novolac-type phenolic resin C in apowder form (particle diameter D₅₀: 28 μm, weight average molecularweight: 3,800, residual carbon ratio: 38 mass %).

Example 1

The carbonaceous substance-coated graphite particles were prepared andevaluated as described below.

Preparation of Graphite Particles

A raw material was pulverized under the conditions (pulverizationconditions) as shown in Table 1 below and granulated, whereby thegraphite particles were obtained. More specifically, flake naturalgraphite (average particle diameter: 8 μm) as the raw material wassequentially passed through 4 pulverizing apparatuses (Hybridizationsystem manufactured by Nara Machinery Co., Ltd.) that were installed inseries. In each pulverizing apparatus, the pulverizing time was 10minutes, and the circumferential speed of the rotor was 60 m/second.

Preparation of Carbonaceous Substance-Coated Graphite Particles

To 100 parts by mass of graphite particles, 3.7 parts by mass of thenovolac-type phenolic resin A was added, followed by mixing at 25° C.for 15 minutes using a drum-type mixer, whereby resin-adhered graphiteparticles were obtained.

The obtained resin-adhered graphite particles were put in a graphitecontainer having a lid and heated with nitrogen flowing at 2 L/min (in anon-oxidizing atmosphere) at 1,200° C. for three hours using a tubularfurnace, whereby carbonaceous substance-coated graphite particles wereobtained.

Each of the physical properties of the obtained carbonaceoussubstance-coated graphite particles was determined according to theforegoing methods. The results are shown in Table 1 below.

Preparation of Negative Electrode

To water, 98 parts by mass of carbonaceous substance-coated graphiteparticles (negative electrode material), 1 part by mass ofcarboxymethylcellulose (binder), and 1 part by mass of styrene butadienerubber (binder) were added, followed by stirring, whereby a negativeelectrode mixture paste was prepared.

The prepared negative electrode mixture paste was applied over copperfoil in a uniform thickness and then dried in vacuum at 90° C., wherebya negative electrode mixture layer was formed. Next, the negativeelectrode mixture layer was pressurized at a pressure of 120 MPa by handpress. Thereafter, the copper foil and the negative electrode mixturelayer were punched out into a circular shape with a diameter of 15.5 mm.A negative electrode adhered to a current collector made of copper foil(coated electrode density: 1.50 g/cm³) was prepared in this manner.

Preparation of Positive Electrode

A positive electrode comprising LiCoO₂ (93 mass %), an electricallyconductive agent (4 mass %), and a binder (3 mass %) was used. Flakegraphite particles and styrene butadiene rubber were used as theelectrically conductive agent and the binder, respectively.

Preparation of Battery for Evaluation

As a battery for evaluation, a button-type secondary battery asillustrated in FIG. 4 was prepared.

FIG. 4 is a cross-sectional view of the button-type secondary battery.In the button-type secondary battery illustrated in FIG. 4 ,circumferential portions of an exterior cup 1 and an exterior can 3 areswaged with an insulating gasket 6 being interposed therebetween,whereby a tightly sealed structure is formed. Inside the tightly sealedstructure, a current collector 7 a, a positive electrode 4, a separator5, a negative electrode 2, and a current collector 7 b are superposed inthis order from the inner surface of the exterior can 3 toward the innersurface of the exterior cup 1.

The button-type secondary battery illustrated in FIG. 4 was prepared asdescribed below.

First, into a mixed solvent comprising ethylene carbonate (33 vol %) andmethylethyl carbonate (67 vol %), LiPF₆ was dissolved so as to have aconcentration of 1 mol/L, whereby a non-aqueous electrolytic solutionwas prepared. A polypropylene porous body (thickness: 20 μm) wasimpregnated with the prepared non-aqueous electrolytic solution, wherebythe separator 5 impregnated with the non-aqueous electrolytic solutionwas prepared.

Next, the prepared separator 5 was held between the negative electrode 2adhered to the current collector 7 b made of copper foil and thepositive electrode 4 adhered to the current collector 7 a made of anickel net so that they were laminated. Thereafter, the currentcollector 7 b and the negative electrode 2 were accommodated in theexterior cup 1, while the current collector 7 a and the positiveelectrode 4 were accommodated in the exterior can 3, and the exteriorcup 1 and the exterior can 3 were put together. In addition, thecircumferential portions of the exterior cup 1 and the exterior can 3were swaged to be tightly sealed, with the insulating gasket 6 beinginterposed therebetween. The button-type secondary battery was preparedin this manner.

Using the prepared button-type secondary battery (battery forevaluation), the battery properties were evaluated though thecharging-discharging test described below. The results are shown inTable 1 below.

In the following charging-discharging test, the process in which lithiumions are absorbed and stored in the negative electrode material isassumed as charging, and the process in which lithium ions are releasedfrom the negative electrode material is assumed as discharging.

Initial Charging-Discharging Efficiency

Constant-current charging at 0.9 mA was performed, and when the circuitvoltage reached 0 mV, the constant-current charging was changed toconstant-voltage charging. Charging was further continued until thecurrent value reached 20 μA. Based on the current amount supplied duringthe foregoing process, the charging capacity per unit mass (unit: mAh/g)was obtained. Thereafter, the battery was rested for 120 minutes. Next,constant-current discharging at the current value of 0.9 mA wasperformed until the circuit voltage reached 1.5 V, and based on theamount of carried current during this process, the discharging capacityper unit mass (unit: mAh/g) was obtained. In addition, the initialcharging-discharging efficiency was calculated by the following equation(1).

Initial charging-discharging efficiency [%]=(dischargingcapacity/charging capacity)×100   (1)

In the test, the process in which lithium ions are absorbed and storedin the negative electrode material is assumed as charging, whereas theprocess in which lithium ions are released from the negative electrodematerial is assumed as discharging.

Charging-Discharging Test: 25° C. Output Resistivity

Constant-current charging at 1.0 C was performed in a 25° C. temperatureatmosphere until the circuit voltage reached 3.82 V. Thereafter, theatmosphere was adjusted to 0° C. temperature atmosphere, and thebutton-type secondary battery was rested for 3 hours.

Discharging at 0.5 C was next performed for 10 seconds, followed by arest of 10 minutes, and charging at 0.5 C was performed for 10 seconds,followed by a rest of 10 minutes.

Next, discharging at 1.00 was performed for 10 seconds, followed by arest of 10 minutes, and charging at 0.5 C was performed for 20 secondsto have the state of charge (SOC) rate of 50%, followed by a rest of 10minutes.

Next, discharging at 1.5 C was performed for 10 seconds, followed by arest of 10 minutes, and charging at 0.5 C was performed for 30 secondsto have the SOC rate of 50%, followed by a rest of 10 minutes.

Next, discharging at 2.0 C was performed for 10 seconds, followed by arest of 10 minutes, and charging at 0.5 C was performed for 40 secondsto have the SOC rate of 50%, followed by a rest of 10 minutes.

After the test, the discharging capacity (unit: mAh) obtained as abovewas multiplied by the respective C rates (0.5 C, 1.0 C, 1.5 C, and 2.0C), whereby the current values were calculated. In addition, a voltage(10-second value) when discharging was performed at each of the C rateswas determined.

The results at the respective C rates were plotted having the currentvalue as the x coordinate and the voltage as the y coordinate, and theinclination of the straight line of linear approximation of theseresults was calculated through least square. The inclination was treatedas the output resistivity (unit: Ω). When this value is smaller, theoutput characteristics can be rated as more excellent.

Furthermore, the 25° C. output resistivity (unit: %) of each example(Examples and Comparative Examples) was obtained from the followingequation. The results are shown in Table 1 below.

25° C. output resistivity [%]=(output resistivity of each example/outputresistivity of Example 1)×100

Charging-Discharging Test: Cycle Characteristic

Constant-current charging was performed at the current value of 1 Cuntil the circuit voltage reached 1 mV and was thereafter changed toconstant-voltage charging. The charging was kept until the current valuereached 20 μA. Based on an amount of current carried during thisprocess, the charging capacity (unit: mAh) was determined. Thereafter,the battery was rested for 10 minutes. Next, constant-currentdischarging was performed at the current value of 2 C until the circuitvoltage reached 1.5 V. Based on an amount of current carried during thisprocess, the discharging capacity (unit: mAh) was determined. Thischarging-discharging operation was repeated 100 times, and the cyclecharacteristic (unit: %) was determined using the obtained dischargingcapacities according to the following equation:

Cycle characteristic=100×(Discharging capacity of 100thcycle/Discharging capacity of 1st cycle)

When this value is larger, the cycle characteristic can be rated as moreexcellent.

Example 2

To 100 parts by mass of the graphite particles, 12.0 parts by mass ofthe novolac-type phenolic resin A was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Example 3

To 100 parts by mass of the graphite particles, 5.7 parts by mass of thenovolac-type phenolic resin B was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Example 4

To 100 parts by mass of the graphite particles, 9.0 parts by mass of thenovolac-type phenolic resin C was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Example 5

The circumferential speed of the rotor of each pulverizing apparatusthrough which the raw material was passed was 100 m/second, and 3.3parts by mass of the novolac-type phenolic resin A was added to 100parts by mass of the graphite particles. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Example 6

The number of the pulverizing apparatuses through which the raw materialwas passed was four, the pulverizing time and the circumferential speedof the rotor of each pulverizing apparatus were 5 minutes and 30m/second, respectively, and 1.5 parts by mass of the novolac-typephenolic resin A was added to 100 parts by mass of the graphiteparticles. Except this change, carbonaceous substance-coated graphiteparticles were prepared in the same manner as in Example 1, and wereevaluated. The results are shown in Table 1 below.

Example 7

The number of the pulverizing apparatuses through which the raw materialwas passed was eight, the circumferential speed of the rotor of eachpulverizing apparatus was 100 m/second, and 3.7 parts by mass of thenovolac-type phenolic resin A was added to 100 parts by mass of thegraphite particles. Except this change, carbonaceous substance-coatedgraphite particles were prepared in the same manner as in Example 1, andwere evaluated. The results are shown in Table 1 below.

Example 8

The circumferential speed of the rotor of each pulverizing apparatusthrough which the raw material was passed was 150 m/second, and 3.9parts by mass of the novolac-type phenolic resin A was added to 100parts by mass of the graphite particles. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Example 9

The number of the pulverizing apparatuses through which the raw materialwas passed was one, the pulverizing time and the circumferential speedof the rotor of the pulverizing apparatus were 5 minutes and 100m/second, respectively, and 11.0 parts by mass of the novolac-typephenolic resin A was added to 100 parts by mass of the graphiteparticles. Except this change, carbonaceous substance-coated graphiteparticles were prepared in the same manner as in Example 1, and wereevaluated. The results are shown in Table 1 below.

Comparative Example 1

To 100 parts by mass of the graphite particles, 45.5 parts by mass ofthe novolac-type phenolic resin A was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Comparative Example 2

To 100 parts by mass of the graphite particles, 20.0 parts by mass ofthe novolac-type phenolic resin C was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

Comparative Example 3

To 100 parts by mass of graphite particles, 15.0 parts by mass ofcoal-tar pitch (residual carbon ratio: 67 mass %) was added, theresultant was heated to 50° C. and mixed by a biaxial kneader for 30minutes, and thereafter heated with nitrogen flowing at 5 L/minute (innon-oxidizing atmosphere) at 1,100° C. for 10 hours using a tubularfurnace, whereby the carbonaceous substance-coated graphite particleswere obtained. Except this change, the carbonaceous substance-coatedgraphite particles were prepared in the same manner as in Example 1, andwere evaluated. The results are shown in Table 1 below.

Comparative Example 4

The number of the pulverizing apparatuses through which the raw materialwas passed was one, the pulverizing time and the circumferential speedof the rotor of the pulverizing apparatus were 20 minutes and 20m/second, respectively, and 3.0 parts by mass of the novolac-typephenolic resin C was added to 100 parts by mass of the graphiteparticles. Except this change, carbonaceous substance-coated graphiteparticles were prepared in the same manner as in Example 1, and wereevaluated. The results are shown in Table 1 below.

Comparative Example 5

The number of the pulverizing apparatuses through which the raw materialwas passed was one, the pulverizing time and the circumferential speedof the rotor of the pulverizing apparatus were 2 minutes and 20m/second, respectively, and 2.0 parts by mass of the novolac-typephenolic resin C was added to 100 parts by mass of the graphiteparticles. Except this change, carbonaceous substance-coated graphiteparticles were prepared in the same manner as in Example 1, and wereevaluated. The results are shown in Table 1 below.

Comparative Example 6

To 100 parts by mass of the graphite particles, 2.7 parts by mass of thenovolac-type phenolic resin C was added. Except this change,carbonaceous substance-coated graphite particles were prepared in thesame manner as in Example 1, and were evaluated. The results are shownin Table 1 below.

TABLE 1 Examples Comparative Examples 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6Pulver- Number of 4 4 4 4 4 4 8 4 1 4 4 4 1 1 4 izing pulverizing con-apparatuses ditions Pulverizing time 10 10 10 10 10 5 10 10 5 10 10 10[[20]] 10 2 10 [min/ apparatus] Total pulverizing 40 40 40 40 40 20 8040 5 40 40 40 20 2 40 time [min] (=number of pulverizing apparatuses ×pulverizing time) Circumferential 60 60 60 60 100 30 100 150 100 60 6060 20 20 60 speed of rotor [m/sec] Total pulverizing ####### ############## ####### ####### 36,000 ####### ####### 30,000 ####### ############## 24,000 2,400 ####### time × circumferential speed of rotor [m]Coating Type Novolac- Novolac- Novolac- Novolac- Novolac- Novolac-Novolac- Novolac- Novolac- Novolac- Novolac- Coal- Novolac- Novolac-Novolac- agent type type type type type type type type type type typetar type type type phenolic phenolic phenolic phenolic phenolic phenolicphenolic phenolic phenolic phenolic phenolic pitch phenolic phenolicphenolic resin resin resin resin resin resin resin resin resin resinresin resin resin resin A A B C A A A A A A C C C C D₅₀ [μm] 34 34 30 2834 34 34 34 34 34 28 — 28 28 28 Weight average 2,800 2,800 1,550 3,8002,800 2,800 2,800 2,800 2,800 2,800 3,800 — 2,800 2,800 2,800 molecularweight Amount of addition 3.7 12.0 5.7 9.0 3.3 1.5 3.7 3.9 11.0 45.520.0 15.0 3.0 2.0 2.7 [parts by mass] Carbona- Amount of 1.3 4.1 1.3 3.41.1 0.5 1.3 1.3 3.7 15.5 7.6 10.0 1.1 0.8 1.0 ceous carbonaceous sub-coatings stance- [parts by mass] coated Specific surface 7.4 12.5 7.86.8 7.3 8.5 7.4 10.5 13.5 17.5 13.2 4.5 3.8 2.1 2.0 graph- area S_(BET)[m²/g] ite Pore volume 0.015 0.013 0.015 0.013 0.024 0.024 0.014 0.0220.014 0.015 0.035 0.016 0.001 0.0005 0.002 par- V_(S) [cm³/g] ticlesPore size P_(max) [nm] 4.1 4.3 4.1 4.0 4.1 4.1 4.4 4.1 4.4 4.1 4.5 6.54.3 4.2 2.1 Volume ratio of 17.5 17.2 18.2 17.4 6.2 51.5 35.0 3.5 15.517.1 17.0 17.9 40 63 18.9 fine grains [%] Volume ratio 36.2 36.7 36.236.5 35.1 45.9 15.5 35.6 65.5 36.2 37.1 36.4 55 71 34 of rod-shapedparticles [%] Bat- Initial discharge 355.1 351.4 355.0 355.4 355.4 356.0355.6 355.0 353.0 350.0 353.0 352.0 355.2 356.1 355.8 tery capacity pro-[mAh/g] perties Initial charging- 94.0 94.0 94.8 92.3 92.4 94.0 93.094.3 93.7 93.0 92.8 91.8 90.3 90.0 90.5 discharging efficiency [%] 25°C. output 100.0 97.4 99.0 98.0 96.8 100.1 100.0 96.0 95.7 110.0 108.0107.0 110.0 130.0 106.2 resistivity [%] Cycle 92.0 92.0 93.0 91.0 91.092.0 91.5 92.3 91.4 89.0 88.0 89.1 90.0 89.0 89.4 characteristic [%]

Summary of Evaluation Results

As Table 1 above shows, Examples 1 to 9 where the specific surface areaS_(BET) was 4.0 to 15.0 m²/g, the pore volume V_(s) was 0.001 to 0.026cm³/g, and the pore size P_(max) was 2.5 to 5.5 nm had better batteryproperties than those of Comparative Examples 1 to 6 where at least oneof these conditions was not satisfied.

REFERENCE SIGNS LIST

1: exterior cup

2: negative electrode

3: exterior can

4: positive electrode

5: separator

6: insulating gasket

7 a: current collector

7 b: current collector

1. Carbonaceous substance-coated graphite particles comprising: graphiteparticles; and carbonaceous coatings covering at least part of surfacesof the graphite particles, wherein the carbonaceous substance-coatedgraphite particles have a specific surface area S_(BET) determined byBET method of 4.0 to 15.0 m²/g, a pore volume Vs of pores with a poresize of 7.8 to 36.0 nm is 0.001 to 0.026 cm³/g, and in a pore sizedistribution graph with the pore size being plotted on a horizontal axisand a dV/dP value obtained by differentiating the pore volume with thepore size being plotted on a vertical axis, a pore size P_(max) withwhich the dV/dP value is maximized is 2.5 to 5.5 nm.
 2. The carbonaceoussubstance-coated graphite particles according to claim 1, wherein in aparticle size distribution of primary particles that is obtained usingX-ray computed tomography, a volume ratio of primary particles with anequivalent spherical diameter of not more than 0.8 μm is 3.0 to 53.0%,and in a particle shape distribution of secondary particles that isobtained using X-ray computed tomography, a volume ratio of rod-shapedsecondary particles is 2.6 to 65.0%.
 3. The carbonaceoussubstance-coated graphite particles according to claim 1, wherein anamount of the carbonaceous coatings is 0.1 to 15.0 parts by mass withrespect to 100 parts by mass of the graphite particles.
 4. Thecarbonaceous substance-coated graphite particles according to claim 1,wherein the carbonaceous substance-coated graphite particles are used asa negative electrode material for a lithium ion secondary battery.
 5. Anegative electrode for a lithium ion secondary battery containing thecarbonaceous substance-coated graphite particles according to claim 1.6. A lithium ion secondary battery including the negative electrodeaccording to claim
 5. 7. The carbonaceous substance-coated graphiteparticles according to claim 2, wherein an amount of the carbonaceouscoatings is 0.1 to 15.0 parts by mass with respect to 100 parts by massof the graphite particles.
 8. The carbonaceous substance-coated graphiteparticles according to claim 2, wherein the carbonaceoussubstance-coated graphite particles are used as a negative electrodematerial for a lithium ion secondary battery.
 9. The carbonaceoussubstance-coated graphite particles according to claim 3, wherein thecarbonaceous substance-coated graphite particles are used as a negativeelectrode material for a lithium ion secondary battery.
 10. Thecarbonaceous substance-coated graphite particles according to claim 7,wherein the carbonaceous substance-coated graphite particles are used asa negative electrode material for a lithium ion secondary battery.
 11. Anegative electrode for a lithium ion secondary battery containing thecarbonaceous substance-coated graphite particles according to claim 2.12. A negative electrode for a lithium ion secondary battery containingthe carbonaceous substance-coated graphite particles according to claim3.
 13. A negative electrode for a lithium ion secondary batterycontaining the carbonaceous substance-coated graphite particlesaccording to claim
 7. 14. A lithium ion secondary battery including thenegative electrode according to claim
 11. 15. A lithium ion secondarybattery including the negative electrode according to claim
 12. 16. Alithium ion secondary battery including the negative electrode accordingto claim 13.